With the popularization and dissemination of 3G and 4G LTE technologies, portable digital products such as laptops, tablet PCs, mobile phones, etc. have become indispensable in people's daily work and life. As a result, demands for portable backup batteries are ever growing. Due to the inability to mate power ports among different digital products, however, backup battery manufactures have to prepare different types or sorts of power sockets to match a user's specific product. This factor can cause high manufacturing costs and may limit the wide utilization of backup batteries.